Having land and sea fortifications always struck me as a pointless duplication that adds little to the game. The number of times where you end up building both land and sea forts in the same province, and the distinction between the two is therefore meaningful, is quite rare.
Yes, I know, the idea is so sea forts won't prevent an attack from a neighbouring province, but since at least HOI3 there are enough provinces in the game that you can simply bypass the coast anyway.